Transparent, biaxially oriented polyester films which feature improved barrier properties are disclosed by the prior art. In most cases, the films obtain their improved barrier layer properties offline after the production by a further processing step. Examples thereof are extrusion coating, coating or lamination with barrier materials, coating under reduced pressure with metals or with ceramic substances, or plasma polymerization in conjunction with vacuum coating.
An exception from this is the process described in detail in WO 99/62694, in which a multilayer, coextruded polyester film which comprises at least one layer of EVOH (ethylene-vinyl alcohol) is simultaneously biaxially stretched. The film features good mechanical properties, but in particular good barrier properties against the passage of oxygen. The best value specified in the document for the achievable oxygen transmission rate (OTR) is 5 cm2/(m2·bar·d). A disadvantage of the process is that regrind obtained in the course of the production cannot be fed back to the process without the film losing its good optical properties.
A further exception is the biaxially oriented film which is described in JP 2001-347592 and consists of a mixture of polyethylene terephthalate and poly(m-xyleneadipamide) (MXD6). The proportion of poly(m-xyleneadipamide) (MXD6) in the film is between 10 and 40% by weight, and the corresponding fraction of polyethylene terephthalate is between 60 and 90% by weight. The film is simultaneously biaxially stretched. It features good mechanical properties, high thermal stability, but in particular a good barrier with regard to the permeation of oxygen. The film achieves an OTR which is less than 30 cm3/(m2·bar·d). In addition, the film features low opacity. The document specifies a value for the opacity which is below 15%. A disadvantage of the film is its two rough surfaces which result from the mixing incompatibility of the two raw materials, polyethylene terephthalate and poly(m-xyleneapdipamide). The high roughness values result in the film losing its original high gloss and obtaining a matt appearance which is undesired for many applications in the packaging industry. A further disadvantage is that the film cannot as efficiently be printed, metalized or coated as films which consist, for example, only of polyethylene terephthalate. The high roughness of the two film surfaces makes the thickness distribution of additionally applied layers (printing inks, metals, varnish, ceramic materials) inadequate. As a result, the physical properties of these layers likewise worsen.
It is an object of the present invention to provide a biaxially oriented polyester film which features very good barrier properties, in particular toward the passage of oxygen. The film should no longer have the disadvantages of the prior art films and should have the following particular features:                at least one of the two film surfaces has roughness values which are so low that the film has, for example, good printability, good metalizability or good coatability, without the effect of this additional functional layer being impaired;        at least one of the two film surfaces has such a high gloss that the film can be used for application in packaging without significant restrictions;        the film can be produced economically, also meaning, for example, that the film can be produced using the stretching processes which are customary in industry, without being restricted to the expensive simultaneous stretching process;        good adhesion between the individual layers of the film has to be ensured for its practical application;        the guarantee in the production of the film that the regrind can be fed back to the extrusion in an amount of up to 60% by weight, without the physical and optical properties of the film, but in particular the barrier toward oxygen, being significantly adversely affected.        
On the other hand, the other properties which are features of polyester films should at the same time not worsen. These include, for example, the mechanical and the thermal properties, the winding behavior and the processibility of the film, in particular in the printing, laminating or the coating with metallic or ceramic materials.